$ B = \left[\begin{array}{rrr}2 & 1 & 0 \\ 0 & 2 & 0 \\ 0 & 0 & 1\end{array}\right]$ What is $ B^{-1}$ ?
$ = \left[\begin{array}{rrr}\frac{1}{2} & -\frac{1}{4} & 0 \\ 0 & \frac{1}{2} & 0 \\ 0 & 0 & 1\end{array}\right]$